Elasmosauridae was the taxonomic family of the most advanced plesiosaurs. They had the longest necks of the plesiosaurs and survived from the Early Jurassic to the end of the Cretaceous.

Size

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The earliest elasmosaurids were small: about 3 m. At the end of the Cretaceous, elasmosaurids grew as large as 14 m., such as Elasmosaurus. Their necks were the longest of all the plesiosaurs.

References

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http://www.palaeos.com/Vertebrates/Units/220Lepidosauromorpha/220.700.html#Elasmosauridae